Conventionally, as shown in FIG. 6, an optical module 50 has an internal waveguide 52 disposed in a groove formed on a surface of a first substrate 51 of both a light-emitting (transmission)-side optical module 50A and a light-receiving (reception)-side optical module 50B, and a mirror portion 53 for optical path conversion formed on a top of the groove.
The surface of the first substrate 51 of the light-emitting-side optical module 50A is mounted with a light-emitting element (optical element) 54A emitting an optical signal to a core of the internal waveguide 52 via the mirror portion 53. In the same manner, the surface of the first substrate 51 of the light-receiving-side optical module 50B is mounted with a light-receiving element (optical element) 54B receiving the optical signal from the core of the internal waveguide 52 via the mirror portion 53.
An external waveguide (optical fiber) 55 is optically coupled to the cores of the respective internal waveguides 52 in the light-emitting element 54A and the light-receiving element 54B.
The light-emitting element 54A or the light-receiving element 54B is flip-chip-mounted via bumps on the surface of the first substrate 51 corresponding thereto, with its mounting surface being a light-emitting surface of the light-emitting element 54A or a light-receiving surface of the light-receiving element 54B. The first substrates 51 are disposed on surfaces of separate second substrates (interposer substrates) 56, respectively. Surfaces of the second substrates 56 are respectively mounted with a signal processing unit (IC substrate) 57A on which an IC circuit for transmitting an electrical signal to the light-emitting element 54A is formed and a signal processing unit (IC substrate) 57B on which an IC circuit for receiving an electrical signal from the light-receiving element 54B is formed. The light-emitting element 54A and the signal processing unit 57A are electrically connected by a looped bonding wire 58, while the light-receiving element 54B and the signal processing unit 57B are electrically connected by another bonding wire 58. The modules 50A and 50B comprise their respective connectors 59 for electrically connecting the signal processing units 57A and 57B to other circuit devices, with the respective connectors 59 and the signal processing units being electrically connected by looped bonding wires 60, respectively.
In such an optical module 50, a conceivable technique for achieving 10 Gbs or more of high-speed transmission is speeding up per wiring line transmitting an electrical signal or multi-channelizing for simultaneously transmitting a plurality of electrical signals.
In the case of speeding up per wiring line, due to an impedance mismatch arising from inductance components of the bonding wires 58, degradation of high-frequency signals by delay induced by a signal reflection becomes not negligible. Since the inductance component is proportional to the length of the bonding wire 58, the length of the bonding wire 58 needs to be shortened for speedup per wiring line.
In the case of multi-channelizing, due to the parallel arrangement of a plurality of wirings, increase in crosstalk as mutual interference of electrical signals between wirings becomes an issue. In general, the crosstalk arises from capacitive coupling between wirings, and measures for its reduction can be for example shortening the wiring length, increasing the wiring interval, and shielding at a GND circuit (ground circuit).
However, demerits occur that the bonding wire 58 makes shielding at the GND circuit infeasible and that increasing the wiring interval results in upsizing of the optical module. Accordingly, even when considering the multi-channelization, the bonding wire 58 needs to be shortened in length.
FIG. 7 shows a sectional view of a bonding-wire-connected light-emitting-side optical module structure 40A-1 as another conventional optical module (see, e.g. Patent Document 1).
In the optical module structure 40A-1, a first substrate 1 has on its surface a recessed step 1g including a slant surface 1e slanting obliquely downward from a vicinity of bumps 12c of a light-emitting element 12a and a horizontal surface 1f extending horizontally from a lower end of the slant surface 1e. 
A metal circuit (circuit patterned by sputtering of copper or gold) 12d connected to the bumps 12c of the light-emitting element 12a is formed along the slant surface 1e and the horizontal surface 1f, with an end of the metal circuit 12d on the horizontal surface 1f being a land 12e. By employing such a configuration, the height position of the land 12e on the horizontal surface 1f can be formed at a low position so as to approximate to the height position of a land 4c on a signal processing portion 4a toward the light-emitting element 12a. 
The land 12e closer to the light-emitting element 12a and the land 4c on the signal processing portion 4a toward the light-emitting element 12a are electrically connected by a looped bonding wire 26.
A land 4d of the signal processing portion 4a associated with a connector 7 and a land 7a of the connector 7 on a surface of a second substrate 6 are electrically connected by a looped bonding wire 27.
The land 7a of the connector 7 is electrically connected, via a through-hole wiring 6a of the second substrate 6, to the connector 7 disposed on a back surface side of the second substrate 6.
In FIG. 7, by lowering the height position of the land 12e closer to the light-emitting element 12a using the recessed step 1g formed on the first substrate 1, a difference H in height from the land 4c of the signal processing portion 4a toward the light-emitting element 12a can be reduced. As a result, the length of the bonding wire 26 can be shortened, so that degradation of the high-frequency signal can be suppressed, enabling a high-speed transmission.